A diffractive optical element that diffracts at least a part of incident light is used in various kinds of optical apparatuses, optical devices or the like. As an example of such an optical device that uses the diffractive optical element, a measuring device may be considered that projects a specific light pattern obtained by diffraction in the diffractive optical element onto a measurement object and measures the shape or the like of the measurement object. A three-dimensional measuring device that measures the three-dimensional shape or the like of a measurement object, and that is an example of such a measuring device, is known as a measuring device that detects a change in a pattern of irradiated light or the like and acquires three-dimensional information (refer to the patent document 1).
Further, if such a measuring device can perform a predetermined measurement even in a case where the distance between the measuring device and the measurement object is short, it is possible to decrease the size of a measuring optical system and to shorten the length of an optical path in alight receiving system, and thus, it is possible to obtain high measurement sensitivity. Thus, in a case where the diffractive optical element is used in such a measuring device, it is preferable that a diffraction angle be large. However, if the pitch of the diffractive optical element is narrowed to generate diffracted light having a large diffraction angle, zero-order diffracted light is generated. Further, if the number of light spots generated by diffractive lights is large, the amount of light of the zero-order diffracted light becomes a value that is relatively larger than the amount of light of the other diffracted lights.
In such a case, there are problems that automatic gain adjustment is set to be low in an imaging device used in the measuring device and it is difficult to recognize light spots generated by the other diffracted lights that are relatively weaker than the zero-order diffracted light. Further, even in a case where a gain is adjusted to be high, there are problems that a blur or the like occurs in the periphery of a light spot generated by the zero-order diffracted light and it is difficult to recognize the light spots generated by the other diffracted lights in the periphery of the light spot generated by the zero-order diffracted light. Thus, in the case of being applied to the three-dimensional measuring device, it is difficult to perform measurement of a three-dimensional shape with high accuracy.
As a configuration for suppressing the occurrence of such a zero-order diffracted light, for example, a configuration in which plural diffractive optical elements are stacked is known, as disclosed in the patent document 2. Further, the non patent document 1 discloses a Dammann diffractive grating that adjusts the light amount of a zero-order diffracted light by adjusting concave and convex shapes formed on a surface of a diffractive optical element.